A preeminent feature of muscle differentiation is the morphogenesis of myofibrils and the onset of contractility. Myofibrillar proteins exhibit impressive self-assembly properties in vitro but the mechanisms governing in vivo events remain uncertain. The experiments described here are designed to elucidate the molecular events involved in the assembly, disassembly and stability of myosin thick filaments in different skeletal and cardiac muscles. A sensitive and quantitative assay for myosin thick filament format on using fluorescence energy transfer (FET) techniques has been established by the applicant. Using this assay and a variety of other biochemical and electron microscopic techniques we propose to: (1) Examine potential differences between myosin isoforms by comparing their assembly characteristics. The extent and kinetics of thick filament formation with myosin isolated from fast and slow skeletal, smooth and cardiac muscles at different stages of development will be examined. (2) We will determine the critical concentration of myosin required to initiate thick filament formation and establish if this is equivalent to the concentration of unassembled myosin present in equilibrium with thick filaments. This will be compared for the different isoforms. (3) The relative stabilities of thick filaments from different muscles will be compared by examining the rates and extents of myosin exchange between filaments. Previous results have indicated that there is extensive exchange of myosin between synthetic thick filaments prepared from adult pectoralis myosin. (4) The mechanism of exchange in synthetic and native thick filaments will also be studied.